P1421: BIMODAL TARGETING OF CYTOKINE RECEPTOR-LIKE FACTOR 2 (CRLF2) WITH JAK INHIBITION AND CHIMERIC ANTIGEN RECEPTOR T CELL IMMUNOTHERAPY IN DOWN SYNDROME ACUTE LYMPHOBLASTIC LEUKEMIA
Authors: A. Bagashev, J. Loftus, L. Niswander, S. Ross, C. D. Falkenstein, J. Junco, K. R. Rabin, T. Fry, S. K. Tasian
Journal: HemaSphere (2022)

Background:CRLF2 (cytokine receptor-like factor 2; encoding the thymic stromal lymphopoietin receptor [TSLPR]) rearrangements occur in 50% of children with Down Syndrome-associated B-acute lymphoblastic leukemia (DS-ALL). These genetic alterations induce constitutive JAK/STAT and other kinase signaling that may be targetable by the JAK1/2 inhibitor ruxolitinib. Patients with DS-ALL experience substantial toxicity with conventional chemotherapy and particularly with hematopoietic stem cell transplantation (HSCT). The Children's Oncology Group AALL1521 clinical trial (NCT02723994) is investigating the potential superior efficacy of ruxolitinib addition to chemotherapy for pediatric patients with CRLF2-rearranged/JAK pathway-mutant BCR-ABL1-like ALL, but children with DS-ALL are not eligible to participate in this study Successful development of precision medicine approaches that can increase cure, reduce treatment toxicity, and avoid HSCT remain a priority for children with D-ALL. Aims: We hypothesized that dual CRLF2 targeting with ruxolitinib and cellular immunotherapy could improve anti-leukemia activity against CRLF2-rearranged (R) DS-ALL. Methods: We previously reported preclinical activity of TSLPR-redirected chimeric antigen T cells with 4-1BB/CD3z costimulatory domains [TSLPRCART] against CRLF2-overexpressing ALL cells (Qin Blood 2015). In the present studies, we investigated the effects of TSLPRCART +/- ruxolitinib co-treatment in vitro and in vivo against CRLF2-R human ALL cell lines and childhood DS-ALL patient-derived xenograft (PDX) models (n=4). Results:In vitro incubation of CD28/CD3 bead-activated normal human T cells or TSLPRCART with ruxolitinib significantly diminished IL-2 and IFN-ɣ cytokine production, although did not induce overt apoptosis of normal or engineered T cells. As expected, treatment of CRLF2-R DS-ALL PDX models with TSLPRCART potently inhibited in vivo leukemia proliferation and induced long-term 'cure' of animals. However, simultaneous co-treatment of engrafted mice with ruxolitinib and TSLPRCART led to markedly diminished in vivo T cell proliferation and persistence, blunted cytokine production, and facilitated eventual leukemia relapse (representative data in Figure 1). These deleterious effects could be abrogated by 14-day delayed sequencing of ruxolitinib after TSLPRCART activation and expansion and were further fully reversible after ruxolitinib withdrawal, resulting in renewed TSLPCART functionality over time and upon leukemia rechallenge. Image:Summary/Conclusion: TSLPRCART is a promising cellular immunotherapeutic approach for DS-ALL and for BCR-ABL1-like ALL given shared features of CRLF2 rearrangement and cell surface protein overexpression. In the present studies, we demonstrate that ruxolitinib co-administration in close proximity impaired in vivo TSLPRCART-induced DS-ALL cell killing but was also beneficial in protection against life-threatening hyperinflammation/cytokine release syndrome (CRS) in dual-treated animals. These results suggest potential for both anti-ALL efficacy and CRS dampening when JAK inhibition and TSLPRCART are properly time-sequenced and may have broader applicability for toxicity mitigation with other cellular immunotherapies. Studies of CAR T cell immunotherapy and other kinase inhibitors in additional CRLF2-rearranged and non-rearranged DS-ALL models are ongoing.